Arrays of biopolymers, such as arrays of peptides or polynucleotides (such as DNA or RNA), are known and are used, for example, as diagnostic or screening tools. Such arrays include regions (sometimes referenced as spots) of usually different sequence biopolymers arranged in a predetermined configuration on a substrate. The arrays, when exposed to a sample, will exhibit a pattern of binding which is indicative of the presence and/or concentration of one or more components of the sample, such as an antigen in the case of a peptide array or a polynucleotide of particular sequence in the case of a polynucleotide array. The binding pattern can be detected, for example, by labeling all potential targets (for example, DNA) in the sample with a suitable label (such as a fluorescent compound), and accurately observing the fluorescence pattern on the array.
Biopolymer arrays can be fabricated using in situ synthesis methods or deposition of the previously obtained biopolymers. The in situ synthesis methods include those described in U.S. Pat. No. 5,449,754 for synthesizing peptide arrays, as well as WO 98/41531 and the references cited therein for synthesizing polynucleotides (specifically, DNA). Such in situ synthesis methods can be basically regarded as repeating at each spot the sequence of: (a) deprotecting any previously deposited monomer so that it can now link with a subsequently deposited protected monomer; and (b) depositing a droplet of another protected monomer for linking. Different monomers may be deposited at different regions on the substrate during any one iteration so that the different regions of the completed array will have different desired biopolymer sequences. One or more intermediate further steps may be required in each iteration, such as oxidation, capping and washing steps. The deposition methods basically involve depositing biopolymers at predetermined locations on a substrate which are suitably activated such that the biopolymers can link thereto. Biopolymers of different sequence may be deposited at different regions of the substrate to yield the completed array. Washing or other additional steps may also be used. Reagents used in typical in situ synthesis are water sensitive, and thus the presence of moisture should be eliminated or at least minimized.
Typical procedures known in the art for deposition DNA such as whole oligomers or cDNA, are to load a small volume of DNA in solution on the tip of a pin or in an open capillary and touch the pin or capillary to the surface of the substrate. When the fluid touches the surface, some of the fluid is transferred. The pin or capillary must be washed prior to picking up the next type of DNA for spotting onto the array. This process is repeated for many different sequences and, eventually, the desired array is formed. Alternatively, the DNA can be loaded into an inkjet head and fired onto the substrate. Such a technique has been described, for example, in PCT publications WO 95/25116 and WO 98/41531, and elsewhere. This method has the advantage of non-contact deposition. Still other methods include pipetting and positive displacement pumps such as the Bio-Dot A/D3000 Dispenser available from Bio-Dot Inc., Irvine, Calif., USA. There are four important design aspects required to fabricate an array of bioplymers such as cDNA's or DNA oligomers. First, the array sensitivity is dependent on having reproducible spots on the substrate. The location of each type of spot must be known and the spotted area should be uniformly coated with the DNA. Second, since DNA is expensive to produce, a minimum amount of the DNA solution should be loaded into any of the transfer mechanisms. Third, any cross contamination of different DNA's must be lower than the sensitivity of the final array as used in a particular assay, to prevent false positive signals. Therefore, the transfer device must be easily cleaned after each type of DNA is deposited or the device must be inexpensive enough to be a disposable. Finally, since the quantity of the assay sample is often limited, it is advantageous to make the spots small and closely spaced.
Similar technologies can be used for in-situ synthesis of biopolymer arrays, such as DNA oligomer arrays, on a solid substrate. In this case, each oligomer is formed nucleotide by nucleotide directly in the desired location on the substrate surface. This process demands repeatable drop size and accurate placement on the substrate. It is advantageous to have an easily cleaned deposition system since some of the reagents have a limited lifetime and must be purged from the system frequently. Since reagents, such as those used in conventional phosphoramidite DNA chemistry may be water sensitive, there is an additional limitation that these chemical reagents do not come in contact with water or water vapor. Therefore, the system must isolate the reagents from any air that may contain water vapor for hours to days during array fabrication. Additionally, the materials selected to construct system must be compatible with the chemical reagents thereby eliminating a lot of organic materials such as rubber.
Given the above requirements of biopolymer array fabrication, deposition using an inkjet type head is particularly favorable. In particular, inkjet deposition has advantages which include producing very small spot sizes. This allows high-density arrays to be fabricated. Furthermore, the spot size is uniform and reproducible as demonstrated by the successful use of inkjets in printers. Since it is a non-contact technique, ink-jet deposition will not scratch or damage the surface. Ink-jets have very high deposition rate, which facilitates rapid manufacture of arrays.
However, an ink-jet deposition system used for fabricating a biopolymer array, should meet a number of requirements. Specifically, the inkjet head must be capable of being loaded with very small volumes of DNA solution and function with minimal or no priming of the inkjets. The system should provide for easy purging of the working solution and readily flushed clean when required. When used for in-situ synthesis, the system should be able to to keep reagents isolated from moisture in the surrounding air. Additionally, use of an inkjet head typically requires that a negative backpressure (that is, a pressure behind an orifice of the jet), in the range of one to six inches of water, be supplied to the inkjet head so that the inkjets form repeatable droplets without dripping during times when the jet has not been activated.
Open-cell foam has been used to provide this negative backpressure in an inkjet printer in a manner disclosed in U.S. Pat. No. 4,771,295, such the capillarity of the foam creates the negative backpressure in an ink reservoir. While this is an easy and economical way to provide the required negative backpressure, the foam cannot be easily purged of the working fluid. A small rubber thimble, similar to an eyedropper, can alternatively be used but the backpressure will vary as the reservoir is depleted. In addition, rubber is incompatible with the chemical reagents typically used in in-situ synthesis. A spring bag reservoir can be designed to control the backpressure of the fluid reservoir, however it requires a large working volume and is therefore not a good choice for the small reservoir volumes required by DNA or other biopolymer array fabrication. A regulated vacuum source could also be used. However, this may permit undesirable components, such as moisture, entering the head particularly during in situ synthesis. Additionally, purging the inkjet head then requires extra valves and a compressed nitrogen (or other suitable gas) source. Gravity is one of the easiest backpressure control means, however the backpressure changes as the fluid height drops and it requires too large a fluid volume to work properly for the small volumes encountered in an inkjet. It would be desirable then, to provide an apparatus and method for fabricating arrays of biopolymers which can use an inkjet type head or other pulse jet head, and which provides for easy purging and cleaning of the head. It would further be desirable that such an apparatus and method provide a simple way of providing a controlled negative backpressure to the head and also provide a simple way of purging the head, without an overly complex system of valves. It would also be desirable that any apparatus and method facilitates isolating reagents in the head from moisture or other undesirable components, and that it is of a compact construction given the small size of other components typically encountered in polynucleotide synthesis.